Sugar (particularly in beverages) and lack of exercise are the main contribution of child obesity increase worldwide.

Don’t blame the potato.

When I was young, in lunch breaks you'd head over to the school's cafeteria. And what they had there? "Gevulde koek", stroopwafel, snack-sized bags of crisps, various (sweetened) dairy products, and iirc bananas & apples too. No veggies or fibre-rich food in sight. So of course kids would go for the sugar-laden & salty snacks. For drinks, there were vending machines. My favorite was hot chocolate. Again: high in sugar (no "more or less sugar" option).

Sometimes there were longer breaks due to a teacher falling ill or whatever. Then we'd head over to a supermarket nearby, and buy similar junkfood.

Of course we'd bring sandwiches from home. But not everyone, and not always. Some kids would throw their sandwiches in a trash can, and proceed to buy the junkfood mentioned above.

That school was 6 years of us kids' lives. Plenty of time to pick up unhealthy habits & bring those into adulthood.

Wouldn't surprise me at all if most schools worldwide are still like that. Healthy diet choices for pupils is far from #1 priority for school management given all the other things they have to deal with.

The fix would be to make healthier options the easy pick, and remove all junkfood offerings from schools' cafeterias. But good luck doing that nationwide under the watchful eye of junkfood multi-nationals.

PS This was one of the major complaints of the long forgotten yellow-vests protests in France.

It's a culture thing.